The invention herein described was made in the course of or under a contract with the Department of the Navy.
This invention relates to systems for receiving wave energy signals and particularly to systems which adaptively suppress interference in the received signal.
Prior art adaptive canceling systems have most often been used in connection with radar systems. A radar system usually uses a highly directional antenna which has a main beam and many sidelobes. A signal within the received frequency band of the radar may cause radar jamming by having sufficient amplitude to saturate the radar receiver when that signal is incident on the antenna sidelobes as well as the main beam. Prior art systems have been developed for cancelling this type of interference in a radar system. These prior art systems usually use an omni-directional auxiliary antenna to receive the interfering signal in an auxiliary channel. The interfering signal in the auxiliary channel is combined with the signal received in the main radar channel to adaptively cancel the interfering signal when the interfering signal is present on the radar sidelobes.
This system minimizes the effects of the interfering signal to only those portions of the radar scanning time when the main beam is pointed at the interfering signal source. This facilitates location of the interfering signal source and prevents that source from interfering with the radar detection of other targets.
The prior art radar adaptive sidelobe canceller usually discriminates between an interfering signal and a desired radar return signal on the basis of signal format. For example, a radar usually transmits short pulses of R. F. energy to facilitate range finding. An interfering signal to be effective is most often a more continuous signal and therefore the adaptive canceling circuit may be designed to respond only to signals having a duration substantially greater than the duration of the desired radar pulse. In other cases the radar signal may be a phase coded or frequency chirped signal which may be distinguished from the undesired interfering signal according to the known characteristics of the desired radar signal.
While useful in system wherein the signal format of the desired signal is easily distinguished from the signal format of the interfering signal, such as radar systems and communications systems using predetermined codes, the above described prior art adaptive sidelobe canceling systems are not usable in systems where there is no distinguishing feature between the desired signal and the interfering signal. For example, in a system where the desired signal is an amplitude modulated signal and the interfering signal is also an amplitude modulated signal in the same frequency band, discrimination on the basis of signal format is not possible. In some of these systems it is possible to distinguish between a desired signal and an interfering signal on the basis of a priori knowledge of the location of the source of the desired signal. Very often sufficient discrimination between the desired signal and the interfering signal may be accomplished using only a directional antenna. In some instances, however, the interfering signal may be of such a greater intensity than the desired signal that the interfering signal received on an antenna sidelobe has greater signal strength in the receiver than the desired signal received on the antenna main beam. This is possible, for example, in an instance where a shore based communications terminal which is seeking to receive a desired signal from ships far at sea is located in the vicinity of another transmitting terminal and receives interference from that transmitting terminal.